1. Technical Field
The present invention relates generally to induction heaters and, in particular, to induction heating systems having switchable power supplies.
2. Background Art
Induction heating is a well known method for producing heat in a localized area on a susceptible metallic object. Induction heating involves applying an AC electric signal to a heating loop or coil placed near a specific location on or around the metallic object to be heated. The varying or alternating current in the loop creates a varying magnetic flux within the metal to be heated. Current is induced in the metal by the magnetic flux, thus heating it. Induction heating may be used for many different purposes including curing adhesives, hardening of metals, brazing, soldering, welding and other fabrication processes in which heat is a necessary or desirable agent or adjurant.
The prior art is replete with electrical or electronic power supplies designed to be used in an induction heating system. Many such power supplies develop high frequency signals, generally in the kilohertz range, for application to the work coil. Because there is generally a frequency at which heating is most efficient with respect to the work to be done, some prior art inverter power supplies operate at a frequency selected to optimize heating. Others operate at a resonant frequency determined by the work piece and the output circuit. Heat intensity is also dependent on the magnetic flux created, therefore some prior art induction heaters control the current provided to the heating coil, thereby attempting to control the heat produced.
One example of the prior art representative of induction heating system having inverters is U.S. Pat. No. 4,092,509, issued May 30, 1978, to Mitchell.
Another type of induction heater in which the output is controlled by turning an inverter power supply on and off is disclosed in the U.S. Pat. No. 3,475,674, issued Oct. 28, 1969, to Porterfield, et al. Another known induction heater utilizing an inverter power supply is described in U.S. Pat. No. 3,816,690, issued Jun. 11, 1974, to Mittelmann.
Each of the above methods to control power delivered by an induction heater either is not adjustable in frequency and/or does not adequately control the heat or power delivered to the workpiece by the heater. The prior art induction heaters described in U.S. Pat. Nos. 5,343,023 and 5,504,309 (assigned to the present assignee) provide frequency control and a way to control the heat or power delivered to the workpiece. These induction heating systems include an induction head, a power supply, and a controller. As used herein induction head refers to an inductive load such as an induction coil or an induction coil with matching transformer.
Some uses of induction heaters are to anneal, case harden, or temper metals such as steel in the heat treating industry. Also induction heaters are used to cure or partially cure adhesives that have metallic particles or are near a metallic part. During the induction heating process a workpiece or part has one or more induction heads placed around and/or in close proximity to the workpiece. Power is then provided to the induction heads, which heat portions of the part near the head, curing the adhesive, or annealing, case hardening, or tempering the part.
One type of power supply used in induction heating is a resonant or a quasi-resonant power supply. As used herein resonant power supply refers to both resonant and quasi-resonant power supplies. A resonant induction heating power supply has an output tank formed by the induction coil or induction head and a capacitor. Current is provided to the tank from a current source and current will circulate within the tank. The current from the current source replenishes the energy in the tank reduced by losses and energy transferred to the work piece. Generally, the tank current facilitates power to the head.
It is desirable in some ways to operate induction heaters at a high frequency output. A higher frequency output allows the magnetic components (inductors and transformers) to be smaller and lighter. This will make the power supply less costly.
The induction heating power supplies described in U.S. Pat. No. 5,343,023 and 5,504,309 have control circuitry that tracks the voltage of the resonant tank, and alternately fires opposite pairs of IGBT's that comprise a full bridge configuration as the tank voltage across the devices transitions through zero. This is an attempt at soft switching, but there is a delay in the control and gate drive circuitry that causes a delay (1.2 .mu.sec e.g.) from the zero crossing until the IGBT turns on. Consequently, when the IGBT turns on, it hard switches into a positive value of voltage and current, and the switching losses become large.
The losses for this sort of power supply increase with frequency. First, as the frequency increases the number of switching events increase. Second, as the frequency increases the 1.2 .mu.sec delay becomes a larger portion of the cycles, and the voltage into which the hard switch is made will be higher. For example, at 10 KHz the voltage will be about 7.5% of the peak after 1.2 .mu.sec: At 50 KHz the voltage will be about 38% of the peak. Thus, the switching voltage is higher and the losses are higher. Finally, conduction losses are greater because the current is off during the 1.2 .mu.sec. The peak current, and hence the RMS current, must be higher to compensate for the time the current is off. Because conduction losses increase with the square of the RMS current, the losses are greater. At higher frequencies 1.2 .mu.sec is a larger portion of the cycle, hence the problem is exacerbated. In sum, higher frequency operation cause three problems: more loss events (more switching), higher losses for each event, and increased conduction losses.
Another prior art resonant power supply described in Chapter 2 of a PH.D. thesis by L. Grajales of Virginia Tech was designed to soft switch a transistor by starting the switching process at zero crossing and then holding the voltage or current, or both, to zero during the turning on and turning off of the transistor. However, this typically required holding the current and/or voltage at zero for a length of time while the switch is turned on. If the propagation delay when turning switches on and off is, for example, 1.2 .mu.sec, this is about 2.4% of the cycle at 10 KHz, and is of little consequence. However, it is 12% of the cycle at 50 hz. Thus, to obtain the desired average current the instantaneous current during the remaining 88% of the cycles must be higher. This requires a higher peak current. In other words, the current must be greater when the current is non-zero to compensate for time it is held to zero (12% at 50 KHz e.g.). This means the peak current is higher, which means the RMS current and losses will also be higher. Thus, soft switching increased conduction losses.
Because soft switching reduces the losses at turn on and turn off, at the expense of increased conduction loss (as described above), it is a design trade off in the Grajales method as to how much duty cycle may be sacrificed in order to achieve minimum switching losses. The practical limit occurs when the increased conduction losses exceed the reduced switching losses.
Accordingly, it would be desirable to provide an induction heating power supply that reduces switching losses without a corresponding increase in conduction losses. Preferably, this would be done by soft switching, or nearly soft switching, the switches used in the output tank. The soft switching will preferably be done by predicting zero crossing and starting the firing process before zero crossing.
The amount of energy delivered to the work piece by the head must be adequately controlled to properly treat the workpiece. This energy depends on, among other things, the energy delivered to the head, the losses in the head, and the relative position of the head to the workpiece (which affects coupling). Some prior art controllers used with inverter based power supplies measure the current delivered to the head. However, in resonant or quasi-resonant induction heaters the resonating current in the tank should be measured.
It is also desirable to be able to determine the tank current so that the user of the equipment knows how much current is flowing in the head and to prevent the capacitors which form the tank from being destroyed by to much current and/or voltage. The current from the current source replenishes the current in the tank due to losses and energy transferred to the work piece.
However, the tank current is high, (1000 amps e.g.) and, to accommodate such high currents, the bus bar through which the current flows is tall, for example a height of 6-18 inches. Thus, it is difficult to obtain current sensing device which will fit around the bus bar. Additionally, mechanical constraints may not allow much room between the bus bars. Accordingly, it would be desirable to have a device which allows current in a resonant tank used in a induction heater to be able to be sensed.
Typically, power supply bus bars (for high current applications) are thin metal plates. Copper bus bars that carry high amounts of current must have the capacity to carry the current without excessive losses (heating). Excessive losses reduce efficiency and increase resistance, thus further increasing losses. Generally, the reference depth and height of the copper plate bus bar determines losses. Thus, the current carrying capacity of a bus bar is increased by increasing its height.
Generally, copper plates have a current carry capacity of about 300 amps for every two inches of height at 60 Hz. However, at high frequencies, such as 50 Khz, the capacity is only about 100 amps per two inches of height. The reduced current capacity is largely due to changed reference depth (which depends on frequency). Thus, prior art 1000 amp induction heaters use a bus bar on the order of 18 inches high. This makes the case much larger than otherwise necessary. Other prior art induction heaters use two inch bus bars that are water cooled. This prevents over heating, but is very inefficient since the losses still occur: they are simply dissipated.
Thus, a bus bar for a 1000 amp induction heater that is efficient yet a reasonable height is desirable.